The present invention relates to a diagnostic device and a diagnostic method of an evaporated fuel processing system, in particular, to an early diagnosis of a leak in the evaporated fuel processing system including a fuel tank.
The present application claims priority from Japanese Patent Application No. 2003-304924, the disclosure of which is incorporated herein by reference.
In order to prevent a fuel evaporated in the fuel tank from being released to the atmosphere, an internal combustion engine including the evaporated fuel processing system is known. In this system, an evaporated fuel (evaporated gas) generated in the fuel tank is temporarily adsorbed by an adsorbent disposed in a canister. Then, the adsorbed evaporated fuel is released to an inlet system of the internal combustion engine through a purge passage under predetermined operating conditions. However, if apart of the system is broken or exploded for some reason, the evaporated fuel is released to the atmosphere. In order to prevent such a situation from taking place, while the evaporated fuel processing system including the fuel tank is being closed, the amount of a change in an internal pressure with elapsed time is monitored so as to execute a leak diagnosis for determining whether there is a leak in the evaporated fuel processing system or not (for example, see Japanese Patent Application Laid-Open Nos. 2001-41116 and 2003-56417).
Moreover, the execution of a so-called early diagnosis prior to the normal leak diagnosis based on the change amount with elapse of the time is also known. The early diagnosis is a method for determining if there is the leak by comparing the internal pressure of the evaporated fuel processing system at certain diagnostic timing with a predetermined criterion threshold value. If it is determined in the early diagnosis that no leak occurs, that is, if the internal pressure of the evaporated fuel processing system is smaller than the criterion threshold value, the subsequent leak diagnosis based on the change amount is cancelled to obtain the result of diagnosis that no leak occurs.
A state of the pressure in the evaporated fuel processing system, however, is not stabilized yet immediately after closing the system because it is affected by an intake negative pressure introduced from the inlet system. A certain time period is required to stabilize the state of the pressure in the evaporated fuel processing system. Accordingly, immediately after the closing, the phenomenon that the internal pressure of the evaporated fuel processing system keeps decreasing with the time below a target value, that is, an overshoot occurs. The degree of the overshoot depends on the intake negative pressure. As the negative pressure becomes deeper, the overshoot becomes larger.
In a conventional early diagnosis, a diagnostic timing is set uniformly and fixedly to the time when a predetermined time period elapsed after completing closing off the evaporated fuel processing system. In this case, it is necessary to set the diagnostic timing in consideration of the case where the largest overshoot occurs. Therefore, according to the conventional method of uniformly setting the diagnostic timing regardless of the degree of the overshoot, it is difficult to optimize the time period required for the early diagnosis in every intake negative pressure area.